The present invention relates to an electric acoustic converter, and more particularly relates to improvement in the tone generating function of an electric acoustic converter such as a speaker generally used for resonance on musical instruments.
A horn-type speaker for vehicles is a typical example of such an electric acoustic converter in which sound waves generated by a vibratory membrane are emanated outside through a horn. (see "Musical Engineering" by Harry F. Olson, McGraw-Hill Book Company, Inc. P183 FIG. 5.77, and P320 FIG. 9, 10) The horn-type speaker is generally provided with vibratory membrane, a horn and an electromagnetic driver unit for the vibratory membrane. The driver unit includes an electromagnet, an armature mechanically connected to the vibratory membrane and a shifter interposed between the electromagnet and a power source. When the electromagnet is not energized, the shifter is spring loaded to be in contact with the armature. When the electromagnet is energized, the same attracts the armature out of contact with the shifter and the electromagnet is disconnected from the power source. Thereupon the armature resumes the original position due to elastic recovery of the vibratory membrane and comes in contact with the shifter again. This process is repeated cyclically for emanation of sound waves through the horn.
This cyclic process is repeated with a resonant frequency fixed by the combination of the vibratory membrane, the armature and the horn. Inasmuch as the driving force acting upon the armature has a complicated wave shape, tones generated by the vibratory membrane are rich in harmonic tones. Under this condition, the resonance characteristics of the horn are quite influential on the tone quality. For generation of comfortable tones, a horn should preferably have a narrow throat maturing into a flare having a soft divergence. A horn having such a configuration is very close in mode of harmonic tone generation to natural musical instruments.
Such a conventional horn-type speaker, however, has a very even sound pressure distribution with respect to frequency (frequency characteristics) and its quality factor (Q) is rather small. As is well known, a small quality factor (Q) leads to low resonance sharpness and, as a consequence, tones generated become very close to those generated by electric and electronic musical instruments.